When you buy content from the iTunes Music Store, it comes in a protected AAC format. This is a form of copy protection (DRM), and it allows you to play your music on up to 5 machines, which is fair. In fact, it's called FairPlay. Before you sell your Mac or give it away, you will want to Deauthorize your Mac via iTunes, so you don't reach your 5 computer limit - Mac or PCs. Here is how to do it: Open iTunes Next, choose the 'Advanced' menu, and select 'Deauthorize computer' Now you can Deauthorize your Music store content & Audible. Be sure to do this, or you will forever be linked to that old machine, even if you have deleted the files.